Polymer-modified asphalt (“PMA”) and methods for preparing various PMA compositions are well known. Polymers, including diene elastomers such as EPDM, EPR and thermoplastic elastomers such as styrene-butadiene and styrene-isoprene block copolymers, are commonly combined with asphalt to improve the performance of the asphalt. It has long been known to add sulfur to the PMA to strengthen and accelerate the polymer-asphalt bonding and curing process. See, for example, U.S. Pat. No. 4,145,322 and U.S. Pat. No. 4,242,246, both listing Paul Maldanado as a co inventor. Various forms of sulfur are used, including elemental sulfur and sulfur-donating compounds. In many cases the sulfur is added as a powder, pellet, flake or in a mixture with other components. For example, U.S. Pat. No. 6,025,418 discloses a process for preparing PMA compositions using a mixture of vulcanizing agents including certain sulfur-donating derivatives, elemental sulfur and an alkaline compound that is soluble in the asphalt.
U.S. Pat. No. 6,569,925 discloses an accelerator-gel additive for use in the production of polymer-modified asphalt. In the system disclosed in the '925 patent, the accelerator-gel additive suspends the accelerator in a gel system preventing the sedimentation of the accelerator during processing. As a result, the polymer and asphalt are processed at lower temperatures, thereby reducing the production of toxic and flammable gases such as hydrogen sulfide. However, there are problems in handling the gel, and it must normally be stored in sealed containers to prevent possible oxidation—not normally a process that is usually available to typical asphalt modifiers.
U.S. Pat. No. 6,767,939 discloses a method for preparing polymer-modified asphalt compositions wherein a certain cross-linking composition is added to the elastomer/asphalt mixture and allowed to cure over a time period of at least forty-five minutes. The cross-linking composition comprises 2-mercaptobenzothiazole, elemental sulfur, zinc oxide, tetramethylthiuram disulfide and stearic acid. However, the composition in the '939 patent requires very low levels of stearic acid to be effective. This has the disadvantage of high generation of hydrogen sulfide.
US Published Pat. Application 2004/0249024 discloses the use of a dispersing agent that facilitates the delivery of a cross-linking agent to polymer-modified asphalt. One of the dispersing agents disclosed in the '024 application is 2-ethylhexyl acid phosphate. Use of such a dispersant has it's own problems, and does not fit well with typical asphalt handling facilities.
What is needed is a process for preparing improved polymer-modified asphalt compositions that can be used in existing asphalt facilities, and does not generate excessive amounts of hydrogen sulfide. Ideally, such a process would be easy to use, stable in storage, and result in quick cross-linking of polymer and asphalt.